The present invention relates to an improvement in a low-pressure mercury vapor discharge lamp, and, more particularly, to a low-pressure mercury vapor discharge lamp having an envelope of a double tube structure which is high in luminous efficacy.
Recently, there has been considerable efforts to provide a fluorescent lamp with a base which is capable of being held and turned on in a socket for the conventional incadescent lamp so as to provide what is called the compact fluorescent lamp.
For example, in U.S. Pat. No. 4,199,708, as shown in FIG. 1, a compact fluorescent lamp having a double tube envelope structure is proposed, comprising a closed lamp envelope 1 and a pair of inner tubes 3, 3' encased in the envelope 1, with an end thereof connected to one end 2 of the envelope 1, the other end thereof being opened into the envelope 1 at openings 7, 7'. The closed lamp envelope 1 is filled with a rare gas of several Torr and a small quantity of mercury, which are adapted to discharge between the electrodes 5, 5' respectively arranged within the inner tubes 3 and 3' whereby ultraviolet rays are generated and converted into a visible radiation by phosphors 4, 4' coated on the inner surface of the inner tube 3, 3'.
In order to prevent the dazzle of radiation of the inner tubes 3, 3', a light-dispersing layer 6 is provided on the inner surface of the closed envelope 1. An operating circuit is disposed in a lamp base 8 having an end provided with a suitable threaded sleeve 9.
Generally, the efficacy of a fluorescent lamp depends on the density N of mercury atoms in the discharge lamp envelope. More particularly, a relationship between the density N of mercury atoms and the partial pressure P of mercury may be expressed as follows: EQU P=kNT (1)
where: k is Boltzmann's constant and T the absolute temperature.
The partial pressure P of mercury coincides with the saturated vapor pressure of mercury at the coldest spot of the discharge lamp. In ordinary fluorescent lamps, the temperature of the coldest spot of the discharge lamp is substantially the same as the glass wall temperature T of the portion where plasma exists, so that the density of mercury atoms in the discharge lamp is almost uniform. The density Nmax of mercury atoms maximizing the efficacy of a fluorescent lamp, which depends on the diameter of the discharge lamp, is generally known to be 1.5.times.10.sup.14 cm.sup.-3 to 3.times.10.sup.14 cm.sup.-3 (which corresponds to the temperature of the coldest tube spot set to 37.degree. C. to 44.degree. C.). Applied Optics Vol. 15, No. 1, 1976, pp 64 to 68).
In the case where the temperature of the coldest spot of the compact fluorescent lamp shown in FIG. 1 is maintained in the above-mentioned temperature range, however, the luminous efficacy of the lamp becomes so low that the lamp cannot be used practically. The luminous efficacy of the lamp is reduced especially when the lamp is intended for high output. This inconvenience is required to be obviated in some ways.